This invention concerns a dither unit for halftoning a contone color image in the form of an array of contone color pixel values, to bi-level dots. In another aspect the invention concerns a method of operating the dither unit.
A general 256-level dither volume provides great flexibility in dither cell design, by decoupling different intensity levels. It provides an extra degree of freedom over a dither matrix when designing an optimal stochastic dither [12]. General dither volumes can be largexe2x80x94a 64xc3x9764xc3x97256 dither volume, for example, has a size of 128 KB. They can also be inefficient to access since each color component can require the retrieval of a different bit from the volume.
The invention is a dither unit for digitally halftoning a contone color image, in the form of an array of contone color pixel values, to bi-level dots. The dither unit comprises a dither volume which contains, for each dither cell location, a fixed set of n thresholds which define n+1 intensity intervals within which the dither cell location is defined to be alternately not set and set.
Such a dither unit may operate to dither each contone color pixel value received at the input port by determining the value of a bi-level output dot for each color component by looking up the dither volume.
A set of thresholds can be used to effectively runlength-encode a bit column of a dither volume, i.e. to compress a bit column. This relies on the fact that there is typically coherence between adjacent bits in the column. When the number of thresholds is limited or fixed, then a general dither volume can no longer necessarily be represented, and so the limited number of thresholds needs to be respected when the dither volume is created. When the number of thresholds is limited to one, a traditional dither matrix is obtained.
A multi-threshold unit may be provided for each color component of the image. All the multi-threshold units are operatively coupled to the dither volume, and each multi-threshold unit determines the value of an output dot corresponding to a contone color pixel component value by determining whether the dither cell location corresponding to the location of the output dot is defined to be set within the intensity interval which the contone value uniquely selects.
Three thresholds may be used, and they may be 8-bit thresholds.
If dither cell registration is desired between color planes, then a multi-threshold value may be retrieved once and used to dither each color component.
If dither cell registration is not desired between color planes, then the dither cell may be split into subcells and stored in separately addressable memories from which different multi-threshold values can be retrieved in parallel.
Where a four color component contone layer is to be halftoned, four separate triple-threshold units may each receive a series of contone color pixel values for respective color components, and a dither cell address generator may operate in conjunction with four four-way multiplexors, for respective triple-threshold units, to control the retrieval of four different triple-threshold values from subcells of the dither unit.
The triple-threshold units may convert triple-threshold values T1, T2 and T3 and an intensity value V into an interval and thence a one or zero bit according to the rules shown in the following Table:
In a further aspect the invention is, in a dither unit for digitally halftoning an contone color image, in the form of an array of contone color pixel values, to bi-level dots, where said dither unit comprises a dither volume, a halftoning method comprising the following steps:
representing each dither cell location of said dither volume by a fixed set of n thresholds which define n+1 intensity intervals within which said dither cell location is defined to be alternately not set and set; and
using the value of the contone pixel component being dithered to uniquely select one of said n+1 intensity intervals, to determine the value of the output dot corresponding to said contone pixel component value.
Where the dither unit further comprises, for each color component of the image, a multi-threshold unit, all of which multi-threshold units being operatively coupled to the dither volume; and the method further comprises the following step:
determining in each said multi-threshold unit the value of an output dot corresponding to a contone color pixel component value by determining whether the dither cell location corresponding to the location of said output dot is defined to be set within the intensity interval which said contone value uniquely selects.